1. Field of the Invention
This invention relates to a color image forming apparatus having a color process cartridge capable of being inserted into and withdrawn from an apparatus body, and discarded when it reaches the limit of use.
2. Description of the Prior Art
The conventional color image forming apparatuses using a transfer drum is disclosed in Japanese Patent Laid-Open No. 61-123257 In the apparatus disclosed in this laid-open publication, a color image is formed by reading a document, modulating a laser beam by a digital signal obtained and writing the information on an image retainer to form an electrostatic latent image thereon, developing this image by a rotary type color developing device, and transferring the toner images of various colors obtained to a transfer paper on a transfer drum sequentially to fix the same thereon Japanese Patent Laid-Open No. 58-72159 proposes a transfer drum type color image forming apparatus having a process cartridge in which an image retaining drum, a plurality of developing devices and a cleaning device are unitarily incorporated This laid-open publication includes a statement to the effect that the apparatus may also have an interchangeable black-and-white image forming process cartridge.
Since the image retaining drum, developing devices and cleaning device are incorporated in the cartridge as stated in Japanese Patent Laid-Open No. 58-72159, a user can prevent an unnatural copying operation which causes scatter of the quality of the images obtained, and the manufacturer can easily guarantee the quality of their products.
However, when it becomes necessary in the apparatus disclosed in this laid-open publication to carry out, for example, the inserting and withdrawing of the process cartridge, the remedying of a jam and the replacement of parts, the image retaining drum and transfer drum have to be separated from each other in the apparatus. This makes it difficult to smoothly carry out the above-mentioned operations, and the necessity of providing a space large enough to carry out the separation of these drums causes the dimensions of the apparatus to increase. Especially, the positioning of the image retaining drum and transfer drum which is done after the completion of the above-mentioned operations of inserting and withdrawing the process cartridge, remedying a jam and replacing parts tends to lack accuracy, so that the imperfect image transfer and the doubling of different color image portions occur.
A cartridge in which an image retaining drum and a transfer drum are arranged is discarded when it reaches the limit of use. In view of the manufacturing cost of the cartridge, it is desirable that the cartridge be discarded on the basis of the lifetime of the image retaining drum.
The lifetime of the image retaining drum is usually supposed to be 50000-80000 in terms of number of color copies taken, and, during the lifetime, a copying operation continues to be carried out by replacing the developing devices and supplementing the toner.
Besides these problems, the waste toner scraped off by a cleaning blade from the image retaining drum, which has been subjected to an image transfer operation, is deposited in the cleaning device every time a copy is taken, and the originally small cleaning chamber shortly becomes unable to hold such waste toner. To eliminate this inconvenience from a conventional apparatus of this kind, a method of transferring the waste toner accumulated in the cleaning chamber, by a screw conveyor to a toner recovery box provided in the apparatus body, or a method of withdrawing a waste toner recovery cylinder inserted in the cleaning chamber is employed to throw away such toner. However, in the case where the waste toner recovery box is used, the insertion and withdrawal of the cartridge become troublesome since it is necessary to transfer the waste toner to the outside of the cartridge by the screw conveyor. In the case where the waste toner recovery cylinder is used, much labor and much copying time are required since it is necessary to interrupt a copying operation every time the recovery cylinder is withdrawn and inserted.
For example, Japanese Patent Laid-Open No. 61-140357 proposes the techniques for recovering the waste toner from an image retaining drum into the hollow therein by a screw conveyor.
With the recent spread of copiers, the development of a light, popular miniaturized machine has been demanded. Accordingly, the miniaturization of the image retaining drum has also been studied, and an image retaining drum having a small diameter of not more than 100 mm has usually been used. Therefore, the substantial capacity of the waste toner-holding hollow in the image retaining drum is small, so that this drum has a practical problem.